1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a structure of a light source device which is suitable for suppressing blurring of a profile of a motion picture (an animated image) displayed on a liquid crystal display panel provided to the liquid crystal display device and for ensuring luminance of a display screen thereof.
2. Description of the Related Art
Recently, mounting of a liquid crystal display device (liquid crystal display module) to a video equipment which displays a so-called motion picture such as a television receiver set or the like has been studied and the movement to sell these equipment in place of video equipment using cathode ray tubes such as Brown tube or the like is actively in progress.
However, compared to the cathode ray tube which displays an image on a screen as an impulse, in the liquid crystal display device which holds an image on the screen every frame period, a profile of an object which moves in the screen every frame period cannot be completely erased every frame period and a strip-like blur is generated along the profile.
On the other hand, a technique which erases an image of previous one frame period from a visual field of a user of the video equipment by periodically turning off a light source device (known as a backlight) which is provided to the liquid crystal display device for every frame period has been studied. Such a technique is described in Japanese Unexamined Patent Publication 2001-108962, Japanese Unexamined Patent Publication 2001-125066 and Japanese Unexamined Patent Publication 2002-123226, respectively. That is, these publications describe the technique which extinguishes a light source of a liquid crystal display device for a fixed period every frame period. However, in this case, since the irradiation of light to a liquid crystal display panel has to be stopped for the fixed period, the luminance of a display screen is lowered. Further, in a light source which irradiates light from an ionized gas generated in a bulb such as a cold cathode fluorescent lamp, a xenon lamp, a fluorescent lamp or the like (hereinafter referred to as “a discharge tube”), due to delay in increasing/decreasing of a light emitting quantity in response to a turn-ON/turn-OFF control of supplying of a lamp current to the discharge tube, even when a light source device provided with the discharge tube is made to perform a blinking operation, a contrast ratio of an image displayed by the liquid crystal display panel is not sufficiently enhanced.
On the other hand, a burst operation method which controls a light emitting quantity by turning on or off a light source device at a period shorter than a frame period is discussed in Japanese Unexamined Patent Publication 11(1999)-299254 and Japanese Unexamined Patent Publication 2000-78857. That is, Japanese Unexamined Patent Publication 11(1999)-299254 describes a technique in which voltage pulses are picked up intermittently from a group of voltage pulses supplied to a driving circuit of a discharge tube in response to burst signals, while Japanese Unexamined Patent Publication 2000-78857 describes a technique in which an alternating electric field which is applied to a discharge tube is intermittently oscillated in response to burst signals. The alternating electric field denotes an electric field having alternating polarity in an extension direction of lines of electric force thereof even if no current appears in the direction.